Visual sensitivity can scale with illusory size changes.

Retinal image size is not the sole determinant of the apparent size of objects. Rather, viewing distance is taken into account when determining apparent size, so images of the same physical dimensions can appear to represent different-sized objects. Here, we take advantage of this to examine the relationship between visual sensitivity and the scaling processes involved in determining apparent size. We assess the impact of illusory size changes, induced by apparent viewing distance changes, on judgments concerning clearly visible stimuli and on the ability to detect low-contrast inputs. We find that sensitivity to slight orientation changes between successive and clearly visible stimuli can scale with illusory size changes. However, illusory size changes have no discernable impact on the ability to detect low-contrast inputs. When considered in conjunction with recent brain imaging studies, our data suggest that visual sensitivity is linked to the spread of activity across primary visual cortex, which for clearly visible stimuli is shaped by the scaling processes involved in the determination of apparent size.